The recent development in a server and a high-end computer cause the central processing unit (CPU) to communicate vastly increasing data with the outer circuits, while the number of pins formed on a package including the CPU is limited for avoidance of electrical interference among the pins. For breaking the limitation of the number of pins, a photoelectric conversion device is provided within the CPU package for use of optical interconnection technology to allow a vast transmission with the outer circuits, that is, the CPU and the outer circuits are optically interfaced.
It is preferable to supply into the market the products adopting both of the optical and electrical connection technology not only the optical connection technology until the optical connection technology will become to be applied to products in the aspects of cost and compatibility to existing devices. Thus, it is desirable for the optical connection technology to be used together with the electrical connection technology.
As the electrical connection technology for connecting an LSI package and a printed circuit board, the ball grid array (BGA) assembling scheme is well known. In this scheme, a plurality of solder balls arranged between the LSI package and the circuit board are melted in a reflow treatment and subsequently solidified to electrically couple the package and the board. Various types of opto-electronic packages are proposed recently as disclosed in Japanese Laid-open Patent Publications 2005-79385 and 2009-139758 and Y. Matsuoka et al., “160 Gbs Compact Optical I/O Module with High Optical Coupling Efficiency”, Proceedings of the 2008 IEICE General Conference, p. 264, 2008.
Japanese Laid-open Patent Publications 2005-79385 has proposed the structure in which a printed circuit board is mounted on an electro optical circuit board via ball electrodes, where the printed circuit board includes a hole for light passing input and output optical signals through the printed circuit board.
Japanese Laid-open Patent Publications 2009-139758 has proposed the structure in which the opto-electronic circuit board is mounted on the board having an optical waveguide via solder balls, where on the opto-electronic circuit board are provided an optical device and an IC chip. The opto-electronic circuit board has a structure like an optical waveguide for optical transmission in the direction of the thickness of the board and a projection for defining the small air gap between the opto-electronic circuit board and the board so as to decrease the optical connection loss.
The reference by Y. Matsuoka et al. has proposed a structure of a lens system improving a tolerance toward a position gap of an optical device, avoiding an optical loss caused by misalign of the optical device, when assembled on the board with an optical waveguide by BGA method.
Each of Japanese Laid-open Patent Publications 2005-79385 and 2009-139758 and the reference by Y. Matsuoka et al. have disclosed that the opto-electronic circuit board is mounted on the board having the optical waveguide by the use of BGA method. Their objects are to decrease the optical loss caused by the air gaps between the opto-electronic circuit board and the board having optical waveguide and by the positional misalignment of optical elements.
For knowing electronic circuit board, means for preventing the scattering of solder flux is disclosed in Japanese Laid-open Patent Publications 05-13938 and 2007-180078. In detail, Japanese Laid-open Patent Publication 05-13938 discloses the method which provides a depressed portion on a solder resist applied to the board as a reservoir of scattered solder flux. Japanese Laid-open Patent Publication 2007-180078 discloses an area applied with the solder resist except a depressed portion the on the board.